1. Field of the Invention
The present invention relates to sensors. More specifically, the present invention relates to systems for measuring the relative position of a ball to a socket for any device using a ball and socket.
2. Description of the Related Art
The ball and socket type gimbal approach has been attractive for many years for pointing and stabilization systems (PSS) for seeker applications. The issue with this approach has been precise control of ball orientation. One of the three primary components in controlling the ball is accurate measurement of ball orientation.
The ball joint gimbal (BJG) employs current art for ball and socket PSS. The basic BJG configuration has a ball in a socket with the ball orientation controlled by four motors that pull on tendon cables attached to the ball. Optical encoders on the motors sense ball orientation by measuring tendon length in an indirect line-of-sight (LOS) measurement approach that restricts the control and stabilization approach.
The BJG has inherently attractive features for seeker applications. For applications with large acceleration requirements, the BJG offers large aperture access and good gun launch hardening characteristics due to the large contact area between the ball and the socket and simplicity in parts. The BJG, however, also has disadvantages that impede its development for small quick munitions. These issues include very complicated processing, ball actuation cable reliability, stabilization for higher frequency excitation, source of small motor/encoders of sufficient accuracy and an excessive time period after power-up for gimbal initialization before use.
The present implementation of the BJG employs remote stabilization by taking angular rate inputs from the inertial measurement unit (IMU) and very accurately measuring the ball orientation at a high data rate to calculate ball inertial position and position rates. This information is used to stabilize ball orientation and LOS. Unlike conventional gimbals, the BJG does not take advantage of a free-floating inner gimbal in which the inertia of the inner gimbal aids in stabilization. The BJG employs rigid coupling between the ball and housing through the actuation cables and motors. Part of this rigid coupling arises because the actuation cables are coiled around the capstans of the motors, creating a mechanical advantage between the motor's rotor and the ball. Small movement of the ball results in large movement of the rotor. Thus, the inertia of the motor's rotor is working against the inertia of the ball through the amplification of the mechanical advantage.
In operation, the orientation of the ball that represents a stabilized LOS for that instance in time is calculated from body rate, and relative ball position inputs, and the ball is driven to that orientation. If the orientation calculation is in error, due to cable stretching, untimely data or other inequities, the ball will be driven to an erroneous orientation causing LOS errors and jitter. Of course this is the case with any gimbal, but the ridged coupling between the ball and the motors prevents the integration or smoothing of high frequency errors by the gimbal's inertia. One result of this iniquity is that as the gimbal excitation increases in frequency, where changes in body rate are not over-sampled by the rate sensor, the error in the orientation calculation increases. The result is that the LOS stability decreases as the excitation increases in frequency which is the opposite of conventional gimbal characteristics.
The measurement of the ball position remotely, i.e. by measuring the actuation cable position, is a natural source for error and requires preflight initialization such that the system knows zero position of the encoders before operation. This calibration takes more time than is desirable in a seeker that must react very quickly. A better way of measuring ball orientation would be to measure it directly at the ball.
Hence, a need exists in the art for an improved system or method for measuring ball orientation in ball and socket type pointing and stabilization systems which is highly accurate, requires no preflight calibration, is small and simple in implementation and measures both axes at once.